Electrolyte permeable diaphragm including a polymeric metal oxide

ABSTRACT

A polymeric metal oxide such as polytitanic acid, polyzirconic acid or polysilicic acid is incorporated into a liquid permeable diaphragm formed of, e.g., asbestos, asbestos and a polymer resin, polytetrafluoroethylene, or a major amount polyfluorocarbon fibrils and a minor amount perfluorinated ion exchange material. Optionally, the diaphragm may also include inorganic materials such as zirconium oxide, titanium dioxide, aluminum oxide, talc, barium sulfate or potassium titanate, and hydrous inorganic gels such as magnesium oxide gel, zirconium oxide gel, titanium oxide gel or zirconyl phosphate gel.

FIELD OF THE INVENTION

The present invention relates to diaphragms useful for the electrolysisof salt solutions, e.g., in the electrolysis of aqueous alkali metalhalide solutions such as sodium chloride brine.

BACKGROUND OF THE INVENTION

Commonly, alkali metal halide brines, such as sodium chloride brines andpotassium chloride brines, are electrolyzed in an electrolytic cellwherein a liquid permeable diaphragm divides the cell into an anolytecompartment with an anode therein and a catholyte compartment with acathode therein to produce chlorine, hydrogen, and aqueous alkali metalhydroxide. Asbestos has been the most common diaphragm material, but hassuffered from relatively short lifetimes and from environmentalconcerns. Numerous efforts have been made to improve the lifetimes andperformances of asbestos diaphragms. For example, according to U.S. Pat.No. 3,991,251, asbestos diaphragms can be strengthened by the reactionbetween asbestos and sodium hydroxide at temperatures from about 110° C.to 280° C. Other patents describe strengthening asbestos diaphragms byaddition of polymeric resins, e.g., fluorine-containing polymers, tobind the asbestos diaphragms. See U.S. Pat. Nos. 4,065,534; 4,070,257;4,142,951; and 4,410,411.

Asbestos-free microporous diaphragms have been produced by sinteringmaterials such as polytetrafluoroethylene (PTFE) and a particulate poreforming additive followed by subsequent removal of the additive, asshown by for example U.S. Pat. Nos. 3,930,979, 4,098,672 and 4,250,002.U.S. Pat. No. 4,036,729 describes depositing discrete thermoplasticfibers of, e.g., a fluorinated hydrocarbon, from an aqueous mediumcontaining acetone and preferably a fluorocarbon surfactant onto acathode screen for use as a diaphragm in electrolytic cells. Thedeposited fibers form an entanglement or network which does not requirebonding or cementing. Unfortunately, such polyfluorocarbon diaphragmsgenerally are hydrophobic, i.e., difficult to wet with water. Thishinders passage of an aqueous electrolyte through the diaphragm, andresults in high cell voltages, particularly in comparison toasbestos-based diaphragms under similar cell conditions.

U.S. Pat. No. 4,482,441 describes codeposition of fibrils of ahydrophobic organic polymer, e.g., a copolymer of tetrafluoroethyleneand perfluoropropylene, and a hydrophilic group IIA metallic oxide,e.g., magnesium oxide particles, from an alkaline brine containingsodium hydroxide, sodium chloride and a polyethyleneimine-basedretention agent onto the cathode of a cell. Such a deposited diaphragmmay also include a surface active agent, e.g., a fluorinated surfaceactive agent.

Finally, U.S. Pat. No. 4,606,805 describes a diaphragm containing as itsprincipal particulate ingredient an inorganic material such as talc, ametal silicate, an alkali metal titanate, an alkali metal zirconate or amagnesium aluminate, along with both polytetrafluoroethylene fibers andpolytetrafluoroethylene particulates.

Clearly, further developments are constantly sought whereby diaphragmsmay achieve improved performance in terms of cell voltages whileexhibiting excellent wettability by aqueous electrolytes.

SUMMARY OF THE INVENTION

The invention herein contemplated provides a liquid permeable diaphragmfor an electrolytic cell, the diaphragm including a polymeric metaloxide exemplified by polytitanic acid, polyaluminic acid, polysilicicacid, and polyzirconic acid. The polymeric metal oxide material isincorporated into the diaphragm by applying a solution including analcohol, water, and a hydrolyzed metal alkoxide, the metal beingselected from aluminum, titanium, zirconium or silicon, to a depositedor preformed diaphragm, and then heating the diaphragm including theapplied solution at temperatures of from about 90° C. to 150° C. to curethe polymeric metal oxide material. In one embodiment, the diaphragmincludes a major amount of fibrillated polyfluorocarbon, e.g.,polytetrafluoroethylene, a minor amount of a perfluorinated ion exchangematerial and a polymeric metal oxide selected from the group ofpolyaluminic acid, polytitanic acid, polyzirconic acid, polysilicic acidor mixtures thereof. For example, the diaphragm may include from about65 to 99 percent by weight fibrillated polyfluorocarbon and from about 1to about 35 percent by weight perfluorinated ion exchange material,basis total weight of polyfluorocarbon and perfluorinated ion exchangematerial. Preferably, the polyfluorocarbon is polytetrafluoroethyleneand the perfluorinated ion exchange material is a perfluorinated organicpolymer containing ion exchange functional groups selected from thegroup consisting of carboxylic acid (--COOH), sulfonic acid (--SO₃ H) oran alkali metal salt of carboxylic acid or sulfonic acid. Suchperfluorinated ion exchange material can be present in the form ofparticulates usually dispersed throughout the diaphragm or as a filmcoating the fibrillated polyfluorocarbon.

Such a diaphragm of fibrillated polyfluorocarbon and perfluorinated ionexchange material may also include a minor amount of inorganicparticulates chemically resistant to the intended cell environment, suchparticulates exemplified by titanium dioxide, zirconium oxide, potassiumtitanate, silicon carbide, aluminum oxide, talc, barium sulfate,asbestos, and mixtures thereof.

In another embodiment, the diaphragm consists essentially of fibrillatedpolyfluorocarbon, e.g., polytetrafluoroethylene, and the polymeric metaloxide as previously described.

In still another embodiment the polymeric metal oxide is included withina liquid permeable diaphragm such as an asbestos diaphragm or anasbestos diaphragm including a polymeric resin.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, a polymeric metal oxide wherein the metal isselected from among titanium, zirconium, silicon, and aluminum orcombinations thereof can be incorporated into a pre-formed diaphragm of,e.g., asbestos, asbestos in combination with a polymeric resin,fibrillated polyfluorocarbon such as polytetrafluoroethylene, or suchfibrillated polyfluorocarbon with perfluorinated ion exchange material.Such polymeric metal oxides are exemplified by polytitanic acid,polyzirconic acid, polysilicic acid and polyaluminic acid. For example,the polymeric metal oxide can be added to a pre-formed diaphragm offibrillated polyfluorocarbon and perfluorinated ion exchange material asa clear solution of a partially hydrolyzed metal alkoxide which prior tohydrolyzation is represented by the formula M(OR)₄ wherein M istitanium, zirconium, silicon, or aluminum, R is an alkyl with from 1 to6 carbon atoms and the solution solvent is an organic solvent for anymetallic or metalloid alkoxide present in the solution. The organicsolvent can be an alcohol such as propanol or ethanol. The clearsolution also contains hydrolyzing water which is generally present inan amount from about 1 mole to about 4 moles per mole of metal alkoxide.In addition the solution may include a few drops of a mineral acid, suchas nitric acid, as a catalyst. Such a clear solution may be allowed toage for up to several hours to provide time for completion ofhydrolyzation whereupon polymerization and cross linking of the metaloxide can occur. The polymeric metal oxide is distributed throughout thediaphragm, e.g., by brushing or spraying the clear solution onto thediaphragm or by dipping the diaphragm into the clear solution tosaturate the diaphragm. Thereafter, the diaphragm is heated attemperatures from about 100° C. to 150° C. for sufficient time,generally about 1 to 2 hours, to dry and cure the polymeric metal oxide.The diaphragm may then be operated in an electrolytic cell.

In similar manner, the polymeric metal oxide may be combined with adiaphragm of fibrillated polyfluorocarbon, e.g.,Polytetrafluoroethylene, in the absence of the perfluorinated ionexchange material. The polymeric metal oxide may provide such adiaphragm with the desired wettability usually provided by theperfluorinated ion exchange material. Generally, the polymeric metaloxide will be present in an amount of from about 1 to about 10 percentby weight, basis total weight of diaphragm.

The polymeric metal oxide may also be added to a diaphragm formed ofasbestos or asbestos in combination with a polymeric resin. Suchasbestos and asbestos-polymeric resin diaphragms can be formed bydeposition of the asbestos and optionally the polymeric resin from,e.g., an aqueous slurry including sodium hydroxide, followed byheat-treating the diaphragm to react the asbestos and sodium hydroxide.Such a heat treatment may be at temperatures whereat the polymeric resindoes not undergo melting or sintering or optionally the resin may bemelted or sintering. The polymeric resin can be chosen from thosedescribed in U.S. Pat. No. 4,186,065 at columns 6 and 7 and suchdescription is hereby incorporated by reference. Other methods ofpreparing asbestos or asbestos and polymeric resin diaphragms are wellknown to those skilled in the art.

The diaphragm separators of this invention are liquid permeable, thusallowing an electrolyte subjected to a pressure gradient to pass throughthe diaphragm. Typically, the pressure gradient in a diaphragm cell isthe result of a hydrostatic head on the anolyte side of the cell, thatis, the liquid level in the anolyte compartment will be on the order offrom about 1 to about 25 inches higher than the liquid level of thecatholyte, although higher or lower levels are permissible andrestricted only by space or electrolytic cell hardware limitations. Thespecific flow rate of electrolyte through the diaphragm can vary withthe type and use of the cell. In a chlor-alkali cell, the diaphragmshould be able to pass about 0.001 to about 0.5 cubic centimeters ofanolyte per minute per square centimeter of diaphragm surface area. Theflow rate is generally set at a rate that allows a predetermined,targeted product concentration, e.g., sodium hydroxide concentration,and the level differential between the anolyte and catholytecompartments is then related to the porosity of the diaphragm and thetortuosity of the pores. For use in a chlor-alkali cell the diaphragmwill preferably have a permeability similar to that of asbestos-typediaphragms so that electrolytic cell equipment in operation withasbestos-type diaphragms can be utilized.

A pre-formed diaphragm of the present invention can be prepared bydepositing the diaphragm material from a slurry onto a liquid permeablesubstrate, e.g., a foraminous cathode. The foraminous cathode iselectroconductive and may be a perforated sheet, a perforated plate,metal mesh, expanded metal mesh, metal rods, or the like. For example,the openings in foraminous cathodes commercially used today inchlor-alkali cells are usually about 0.05 to about 0.125 inches indiameter. Most commonly the cathode will be of iron or an iron alloy. Byiron alloy is meant a carbon steel or other alloy of iron.Alternatively, the cathode can be nickel or other cell environmentresistant electroconductive material. Cathodes suitably used in thisinvention include those having an activated surface coating, forexample, those cathodes with a porous Raney nickel surface coating.Raney nickel coatings can provide a reduction of hydrogen overvoltage atthe cathode and allow a savings in energy consumption and cost in theelectrolysis of brine. Raney nickel coatings can be provided by variousexpedients well known to those skilled in the art.

Such diaphragms are generally deposited upon the foraminous cathode inan amount of about 0.1 to about 0.5 pounds per square foot diaphragmmaterial more preferably about 0.25 to 0.35 pounds per square footdiaphragm material, e.g., asbestos, polyfluorocarbon fibrils,perfluorinated ion exchange material, etc. The diaphragm will generallyhave a thickness of about 0.01 to 0.25 inches, preferably about 0.02 to0.15 inches to achieve best results in terms of voltage and energyefficiency.

The pre-formed diaphragm of this invention can include fibrillatedpolyfluorocarbon and optionally perfluorinated ion exchange materialwherein such diaphragm is prepared by depositing any perfluorinated ionexchange material in the form of discrete particulates or as a solution,and polyfluorocarbon fibrils from a slurry onto a cathode, e.g., onto acathode with a non-planar configuration. For example, polyfluorocarbonfibrils and discrete perfluorinated ion exchange material particulatescan be dispersed within the liquid slurry without rapid settling withsurfactants and viscosity modifiers added to aid in the dispersion.Following deposition, a fibrillated polyfluorocarbon mat having a highlybranched structure, which branched structure provides support for thediaphragm through entanglement of the fibrils, is formed. Thepolyfluorocarbon fibrils can be drawn against the cathode under thepressure of a vacuum to provide packing of the diaphragm material.

Inclusion of perfluorinated ion exchange material with thepolyfluorocarbon fibrils can provide the diaphragm with wettability,i.e., an aqueous brine can pass through the diaphragm without thenecessity of first passing a liquid such as an alcohol through thediaphragm. Also, such a diaphragm will not tend to accumulate gasbubbles and thus may maintain low steady voltages. Perfluorinated ionexchange material may serve additionally as a glue or binder for thefibrils. Generally, such a diaphragm will contain a major amount of thepolyfluorocarbon fibrils, i.e., greater than 50 percent by weight of thefibrils. As perfluorinated ion exchange material is generally morecostly than polyfluorocarbon fibrils, the diaphragm more preferablyincludes from about 65 to about 99 percent by weight polyfluorocarbonfibrils and from about 1 to about 35 percent by weight perfluorinatedion exchange material. Within such percentage ranges, the largerpercentages of polyfluorocarbon fibrils are most preferred to minimizediaphragm cost, i.e., the diaphragm includes from about 95 to about 99percent by weight polyfluorocarbon fibrils and from about 1 to about 5percent perfluorinated ion exchange material wherein the perfluorinatedion exchange material provides the diaphragm with wettability.

Fibrillated polyfluorocarbon materials useful in this invention include,for example, polyvinylfluoride, polyvinylidene fluoride,polyperfluoro(ethylene-propylene), polytrifluoroethylene,poly(chlorotrifluoroethylene-ethylene),poly(tetrafluoroethylene-ethylene), polychlorotrifluoroethylene, andpolytetrafluoroethylene. Preferably, the polyfluorocarbon ispolytetrafluoroethylene (PTFE).

Perfluorinated ion exchange material may be incorporated in a diaphragmof this invention in the form of, e.g., a solid, a gel or a solution. Asa solid, for example, the perfluorinated ion exchange material can beadded to the slurry as discrete particulates or fibers. As a solution,perfluorinated ion exchange material can be added to the slurrydissolved in any suitable solvent such as ethanol although rather thanbeing dissolved the perfluorinated ion exchange method may be highlysolvated particles. The solid perfluorinated ion exchange material maybe, e.g., in the acid form of the perfluorinated ion exchange materialand may be swollen with an organic liquid such as ethanol orisopropanol.

Such perfluorinated ion exchange material is generally an organiccopolymer formed from polymerization of a fluorovinylether monomercontaining a functional group, i.e., an ion exchange group or afunctional group easily converted into an exchange group, and a monomerchosen from the group of fluorovinyl compounds, e.g., vinyl fluoride,vinylidene fluoride, trifluoroethylene, tetrafluoroethylene,hexafluoropropylene, chlorotrifluoroethylene, andperfluoro(alkylvinylether) with the alkyl being a C₁ -C₁₀ alkyl group.The functional groups are --COOM or --SO₃ M or may be --PO(OM)₂ or--OPO(OM)₂ where M is hydrogen or an alkali metal ion. Further, thefunctional groups may be precursors of the --COOM or --SO₃ M groupswhich can be converted to the carboxylic acid or sulfonic acid and saltsthereof by hydrolysis.

The content of the fluorovinylether having the functional groups in thecopolymer is important as it determines the ion exchange potential ofthe perfluorinated ion exchange material and thus, controls itshydrophilicity or wettability. The fluorovinyl ether content isgenerally in the range of about 1 to about 50 mole percent, preferablyabout 2 to about 40 mole percent. Generally, the equivalent weight ofthe perfluorinated ion exchange material will be from about 600 to 2000.Equivalent weight is the weight of material in grams which contains oneequivalent of potential ion exchange capacity.

The perfluorinated ion exchange material can generally be from thosematerials presently supplied for use as electrolyte impermeablemembranes in various electrolytic cells, in particular, the membranematerials known as Nafion®, available from E. I. DuPont de Nemours andCompany and those known as Flemion®, available from Asahi Glass Company,Ltd.

In a preformed diaphragm of fibrillated polyfluorocarbon andperfluorinated ion exchange material, the diaphragm may further includea minor amount of chemically resistant inorganic particulates e.g.,particulates selected from the group of zirconium oxide, titaniumdioxide, potassium titanate, aluminum oxide, silicon carbide, talc,asbestos, barium sulfate and mixtures thereof. Such diaphragms maycontain from about 70 to about 95 percent by weight fibrillatedpolyfluorocarbon, e.g., polytetrafluoroethylene, from about 1 to about 5percent by weight of the perfluorinated ion exchange material, i.e., anamount sufficient to provide wettability, and a minor amount of theinorganic particulates, i.e., from about 1 to 25 percent by weight, morepreferably from about 5 to 15 percent by weight inorganic particulates,basis total weight of diaphragm.

It may be desirable and even preferable that the pre-formed diaphragm beasbestos-free. Thus, the polymer metal oxide would be incorporated intoan asbestos-free diaphragm, e.g., a diaphragm of fibrillatedpolyfluorocarbon. Also in those diaphragms including fibrillatedpolyfluorocarbon, e.g., polytetrafluoroethylene, it may be preferable touse unsintered polytetrafluoroethylene to form the diaphragm. Suchunsintered, fibrillated polytetrafluoroethylene may be preferred overfibrillated polytetrafluoroethylene that has been sintered at some stageprior to fibrillation.

The liquid permeability of the diaphragms can be adjusted by utilizationof a pore forming material, inorganic gels or combinations thereof. Forexample, a pore forming material can be included, e.g., codeposited withpolyfluorocarbon fibrils and perfluorinated ion exchange material. Suchpore forming material is subsequently removable, e.g., by chemicalleaching after formation of the diaphragm, by heating to decompositiontemperatures of the pore forming material following formation of thediaphragm, or by removal in situ during subsequent operation of the cellvia the chemical action of an electrolyte within the cell. Amongsuitable pore formers in the preparation of the diaphragms arecellulose, rayon, polypropylene, calcium carbonate, starch, polyethyleneand nylon. Cellulose, rayon, polypropylene, polyethylene or nylon can bepresent in any suitable particulate form, e.g., granular or fibrousform. Preferably, the pore forming material is polyethylene orpolypropylene and present in fibrous form. Generally, the pore formingmaterial can be added in an amount from about 1 to about 30 percent byweight, more preferably from about 1 to about 20 percent by weight,basis total weight of diaphragm materials.

The diaphragm can also incorporate an inorganic gel. The inorganic gelmay be a hydrous metal oxide gel such as magnesium oxide gel, zirconiumoxide gel, or titanium oxide gel, a zirconyl phosphate gel, orcombinations thereof. Such inorganic gels can serve to reduce the liquidpermeability of a diaphragm to a desired level and may also provide ionexchange properties to the diaphragm. The inorganic gel is added to thediaphragm after formation of the diaphragm and preferably after thepolymeric metal oxide is incorporated into the diaphragm. For example,after a diaphragm of fibrillated polytetrafluoroethylene andperfluorinated ion exchange material is formed upon a non-planar cathodeand the polymeric metallic oxide is incorporated into the diaphragm inaccordance with the present invention, an inorganic gel can be added tothe diaphragm matrix by filling the matrix with an inorganic gelprecursor, i.e., a solution of an inorganic salt, e.g., zirconiumoxychloride, titanium oxychloride, or magnesium chloride and thereafter,hydrolyzing the inorganic salt thereby providing a hydrous oxide of thezirconium, titanium or magnesium as the inorganic gel. Magnesium andzirconium inorganic gels can be prepared, e.g., in the manners describedin U.S. Pat. Nos. 4,170,537, 4,170,538 and 4,170,539. A zirconylphosphate gel can be formed by filling the diaphragm matrix with asolution of zirconium oxychloride and then contacting the matrix with asolution of dibasic sodium phosphate to precipitate zirconyl phosphategel.

Precursors of such hydrous inorganic gels can be deposited in variousways. For example, a solution of the precursor can be brushed or sprayedonto the diaphragm matrix if the solution will penetrate or soak intothe porous matrix. Otherwise, the diaphragm matrix can be immersed inthe solution, a vacuum drawn to remove the air from the matrix and thevacuum released to draw the solution into the matrix.

Inorganic gels can also be incorporated in the diaphragm in situ duringcell operation. For example, an inorganic salt such as magnesiumchloride hexachloride or zirconium oxychloride can be added to anolyte,i.e., the brine feed, while the diaphragm is operated in a chlor-alkalicell whereby an inorganic gel can be formed within the diaphragm poresin situ. Mixtures of inorganic salts may be added. Preferably, theinorganic salts may be added to the anolyte immediately after cellstartup, i.e., within the first few hours, more preferably, first fewminutes, in the period before the hydroxide ions formed at the cathodehave begun to migrate substantially through the diaphragm towards theanode.

In operation of chlor-alkali cells containing the diaphragms of thisinvention, sodium chloride brine feed generally containing from about290 to 330 grams per liter of sodium chloride will be fed to the anolytecompartment. Such a brine feed can have a quality similar to thattypically used in asbestos-type diaphragm cells, i.e., the brinegenerally can contain about 2 to 3 parts per million alkaline earthmetal ion impurities such as calcium and magnesium. In some instances,it may be desirable to use higher quality brine, i.e., brine containingless than about 20 parts per billion alkaline earth metal impurities.Brine treatment methods capable of obtaining the desired quality levelsare well known to the skilled in the art.

The present invention is more particularly described by the followingexample which is intended as illustrative only, since numerousmodifications and variations will be apparent to those skilled in theart.

Example 1

Polytetrafluoroethylene powder (TEFLON®60, available from E. I. DuPontdeNemours and Co.) was blended with granular sodium chloride in aBrabender mixer at a PTFE:salt weight ratio of 1:10. The resultant clumpwas removed from the mixer and chopped in a blender to break up theclump. The salt was dissolved in water and polytetrafluoroethylenefibrils of about 20 to 250 microns in diameter and about 1 to 4millimeters in length were washed, dried and recovered. A mixtureincluding 8.6g of the PTFE fibrils, 0.96g of polypropylene fibers(POLYWEB® available from James Rivers Corporation), 1.25g of an ethanolsolution of a perfluorinated ion exchange material having sulfonic acidfunctional groups, 8.3 weight percent of the ion exchange material(Nafion 601®, available from E. I. DuPont de Nemours and Co.), 4.0 g ofRHEOTHIK® 80-11 viscosity modifier and 4.0 g of a non-ionic surfactant(a polyethoxylated aliphatic chloride, i.e., C₁₀₋₁₅ (OCH₂ CH₂)₉ Cl) wasblended in about 225 ml of water.

The slurry was poured over a 3 inch by 3 inch perforated steel platecathode covered with cellulose filter paper and a 25 inch mercury vacuumwas applied to draw the slurry liquids through the cathode. The solidswere filtered out as a mat atop the filter paper. The cathode anddiaphragm mat were placed in an oven and dried at temperatures between120° C. to 130° C. for 30 minutes with continued application of thevacuum.

After the diaphragm mat cooled, the diaphragm was impregnated with aclear solution of both partially hydrolyzed silicon alkoxide andzirconium alkoxide. The clear solution was formed by adding 10 gtetraethoxysilane (Si(OC₂ H₅)₄) to 100 g of 2-propanol. To this mixturewas added 0.87g water and four drops concentrated nitric acid, followedby stirring for 30 minutes at 60° C. Then 15g Zr(OC₃ H₇)₄ was added andthe mixture stirred for 5 minutes. Another 50g of 2-propanol was added,followed by the addition of 2.6g water in 16g 2-propanol. Finally,another 25g 2-propanol was added. The diaphragm was impregnated with thesolution by dipping in the solution. A vacuum was drawn on theimpregnated diaphragm to maintain an air flow through the diaphragmthereby maintaining permeability and the diaphragm was heated for twohours at 120° C. A second coat of the solution was applied via dipping.

The cathode-diaphragm assembly was then placed into a laboratorychlor-alkali cell having a ruthenium oxide/titanium oxide coatedtitanium mesh anode. The cell was operated with the anode against thesurface of the diaphragm. The cell was fed a purified sodium chloridebrine (25 weight percent NaCl) containing less than 20 parts per billiontotal of calcium and magnesium. The cell was operated at about 90° C.with a current density of 133 amperes per square foot (ASF) produced10.4 weight percent sodium hydroxide (125 gpl) at 2.82 volts and with acathode current efficiency of 92.6 percent.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent they are included in the accompanying claims.

What is claimed is:
 1. A liquid permeable diaphragm for an electrolyticcell, the diaphragm comprising a major amount of fibrillatedpolyfluorocarbon, a minor amount of a perfluorinated ion exchangematerial and a polymeric metal oxide selected from the group consistingof polytitanic acid, polyzirconic acid, polysilicic acid, polyaluminicacid or mixtures thereof.
 2. The diaphragm of claim 1 wherein theperflourinated ion exchange material is a perfluorinated organic polymerhaving ion exchange functional groups selected from the group consistingof --COOM and --SO₃ M where M is hydrogen or an alkali metal ion.
 3. Thediaphragm of claim 2 wherein the fibrillated polyfluorocarbon comprisespolytetrafluoroethylene.
 4. The diaphragm of claim 3 wherein thepolymeric metal oxide is applied to a predisposed diaphragm mat of thefibrillated polytetrafluoroethylene and the perfluorinated ion exchangematerial by impregnating the mat with a solution of a metal alkoxide, analcohol and water, the metal being titanium, aluminum, zirconium,silicon or mixtures thereof and heating the impregnated mat attemperatures and for a period of time sufficient to cure the polymericmetal oxide.
 5. In a liquid permeable diaphragm for an electrolyticcell, the improvement wherein the diaphragm further includes a polymericmetal oxide selected from the group consisting of polytitanic acid,polyaluminic acid, polyzirconic acid, polysilicic acid or mixturesthereof.
 6. The diaphragm of claim 5 wherein the diaphragm is anasbestos diaphragm.
 7. The diaphragm of claim 5 wherein the diaphragmincludes asbestos and a polymeric resin.
 8. The diaphragm of claim 5wherein the diaphragm is an asbestos-free diaphragm consistingessentially of polyfluorocarbon.
 9. The diaphragm of claim 8 wherein thepolyfluorocarbon is polytetrafluoroethylene.
 10. A liquid permeablediaphragm for an electrolytic cell consisting essentially of fibrillatedpolyfluorocarbon and a polymeric metal oxide selected from the groupconsisting of polytitanic acid, polyaluminic acid, polyzirconic acid,polysilicic acid or mixtures thereof.
 11. The diaphragm of claim 10wherein the polymeric metal oxide is applied to a predisposed mat of thefibrillated polyfluorocarbon by impregnating the mat with a solution ofa metal alkoxide, an alcohol and water, the metal being titanium,aluminum, zirconium, silicon, or mixtures thereof and heating theimpregnated mat at temperatures and for a period of time sufficient tocure the polymeric metal oxide.
 12. The diaphragm of claim 10 whereinthe fibrillated polyfluorocarbon is polytetrafluoroethylene.
 13. Thediaphragm of claim 11 wherein the fibrillated polyfluorocarbon ispolytetrafluoroethylene.